It's the nepo babies in Hollywood lmao. Even the Game of Thrones showrunners are nepotism hires. Writing decent fantasy is fairly easy when you stop and take a moment to think about the implications of something. For example, if domesticated dragons are real, then they become WMDs and warfare is going to center around how to avoid their use and how to defend against them.

A lot of these failed projects aren't stopping to ask these kind of questions. Instead, they imitate successful franchises, eliminate the human storytelling, and create plot holes/remove the suspension of disbelief when they ignore how their fantasy world works.